1. Field of the Invention
The present invention relates to a diamond heterojunction diode to be used for a semiconductor rectifying device.
2. Description of the Prior Art
Diamond has a high thermal conductivity, an excellent stability against heat, and a large band gap. It is electrically insulating but becomes semiconducting upon doping. Therefore, diamond is expected to be used for semiconducting devices which can function in a high electric power/high temperature region. By the advent of the synthesis of diamomd films by chemical vapor deposition (CVD), it has now become possible to grow B (boron)-doped p-type semiconducting diamond films and Si-doped or P (phosphine)-doped n-type semiconducting diamond films.
Research and development effects have been made to develop semiconductor devices employing such semiconducting diamond films, for example, a diamond p-n junction diode as a rectifying device. Such a diamond p-n junction diode has been disclosed in the literature (Iwasaki et al., Abstract of the Spring Meeting of Japan Society of Applied Physics, 30a-ZB-10, p. 388 (1990)), wherein the diamond p-n junction diode is obtained by growing a P-doped n-type semiconducting diamond layer on a B-doped p-type semiconducting diamond layer, thereby forming the p-n junction having the rectifying effect.
FIG. 6 is an energy band diagram for the conventional diamond p-n junction diode: (a) is under zero bias (bias voltage V=0); (b) is under a forward bias (V&gt;0) where a positive voltage is applied to the p-type semiconducting diamond layer P; and (c) is under a reversed bias (V&lt;0) where a negative voltage (the n-type semiconducting diamond layer N is positive) is applied to the p-type semiconducting diamond layer P. In this figure, E.sub.c is the energy at the lowest edge of the conduction band, E.sub.v is the energy at the highest edge of the valence band, and E.sub.F is the Fermi level.
At present, the CVD technology makes possible the synthesis of a p-type semiconducting diamond film with a resistivity as low as about 10.OMEGA..cm. However, it merely permits the synthesis of a n-type semiconducting diamond film with a resistivity of the order of 10.sup.4 -10.sup.6 .OMEGA..cm because only a low concentration of active impurities can be contained therein. For this reason, in the conventional diamond p-n junction diode, the depletion layer becomes thick on the n-type semiconducting diamond layer side of the p-n junction. Consequently, the curvature of the energy band at the p-n junction is dull, and hence the gradient of electrical potential at the p-n junction is not sharp as shown in FIG. 6.
Therefore, under a forward bias, when holes are transported from the p-type layer into the n-type layer or electrons from the n-type layer to the p-type layer, the transport velocity is slow and the travel distance becomes longer across the p-n junction. As a result, these carriers are liable to be trapped by lattice defects and dopant atoms and disappear due to recombination therewith, thus preventing a flow of a sufficient electric current. Furthermore, there often yield amorphous layers and defects at the interface of the p-n junction, which lower the barrier height under the reversed bias, thus allowing a current to flow in the reversed direction. Therefore, such a diamond p n junction diode needs improvement in its rectifying characteristics.